Seasonal skull reduction

dechmann-et-al-2017Braincase shrinkage during winter was firstly described in shrews by Dehnel in 1949, and is known as the Dehnel’s Phenomenon. Recently, Dechmann et al. investigated the seasonal size variation in the skulls of shrews (Sorex araneus) and least weasels (Mustela nivalis). They measured skull length and braincase depth on specimens previously collected from a Polish National Park, sampling all seasons. Both species showed an initial juvenile growth until the first winter, followed by shrinkage until spring in the adults, and a subsequent re-grow on the second summer, though never reaching the initial size. Heat maps built from high resolution CT scans demonstrated that size changes also involved changes in shape and in bone thickness, with the thinnest skulls coinciding with the smallest braincase size. Interestingly, these patterns differed between sexes, especially in weasels as only males were observed to re-grow. Despite phylogenetically distant, both species have similar life histories, having short life spans and high metabolisms, and inhabiting an environment with seasonal fluctuation of resources availability. Winter shrinkage would reduce energetic requirements and prepare individuals for the harder conditions, and re-growth during the resources-abundant season would prepare the males for reproduction while females would allocate the energy into caring for the offspring.  The authors conclude these seasonal reversible size changes are genetically fixed and exclusive of animals with such life histories, as an adaptation to extreme environmental conditions. Future investigation shall clarify the potential drivers and consequences of this phenomenon, including how the variation in size affects brain size and reorganization.

Sofia Pedro

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One response to “Seasonal skull reduction

  • sofiappedro

    Other studies have been focusing on the impact of climate change on skull morphology. Assis and colleagues analyzed 20 species of Tamias and Neotamias chipmunks, covering a wide range of habitats across North America, from alpine tundra to temperate rainforests. They found that genetic drift alone was insufficient to explain the diversification of cranial morphology and that this was mostly influenced by one environmental variable, which was the minimum temperature of the coldest month. The smallest chipmunks inhabit colder regions, such as high-elevation alpine habitats, while the largest species inhabit the more moderate coastal areas. And species in hotter, wetter climates exhibit narrower neurocrania with longer faces, while species in colder, dryer climates have shorter faces and wider neurocrania.
    On another study, Walsh et al. found that, in response to changes in climate conditions over the last century, the alpine-specialized Tamias alpinus experienced greater changes in diet and craniofacial morphology than the more broad-distributed Tamias speciosus. The two studies evidence the impact of environmental conditions in the species’ morphology and highlight the role of the cold temperatures as a selective force.

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